As a basic measurement tool, the oscilloscope generally has a grid or graticule superimposed on the display screen which is divided into one-centimeter or one-half-inch divisions. The horizontal sweeping of the cathode-ray tube beam is generally calibrated to the graticule so that sweep rates of time units per graticule division may be established. Differential time measurements between two points of interest were made within the accuracy of human judgment using the graticule scale, or a ruler could be used to measure between the points of interest if a greater accuracy was desired.
In oscilloscopes having delaying and delayed sweep capability, the Delay Time Multiplier control can be utilized to provide differential time measurements. This control is typically a linear 10-turn potentiometer having a mechanical arrangement of dials thereon to provide a reading of the time position of a point on the sweep.
One scheme that was devised to simplify time interval measurements was the two-dot system disclosed in U.S. Pat. application Ser. No. 371,220 filed June 18, 1973. This system included two controls, one for moving both dots along the sweep, and the other for controlling the separation between the dots. The separation, or time interval, between the dots was read out on the mechanical dial described above.
The first scheme to provide an electrical readout of the time interval was that disclosed in U.S. Pat. applicaton Ser. No. 532,089 filed Dec. 12, 1974, wherein two delay pickoff comparators were employed to facilitate a dual delayed sweep. A voltage proportional to the difference between the two preselected comparator levels was applied to a digital voltmeter scaled to provide a time reading.